Monostable multivibrator employing capacitive feedback circuit maintaining voltage across input circuit substantially constant



Aprll 19, 1966 A, NoYEs, JR 3,247,458

. MONOSTABLE MULTIVIBRATOR EMPLOYING CAPACITIVE FEEDBACK CIRCUITMAINTAINING VOLTAGE ACROSS INPUT CIRCUIT SUBSTANTIALLY CONSTANT OriginalFiled July 1, 1948 FEM H628 INPUTSIGN 30b TUBE so PLATE BACK FIGZC V VTUBE36AGR1D FIGZD n n TUBE 36A PLATE INVENTOR ATHERTON NOYES JR.

BY M g ATTORNEYS United States Patent Ofiice 3,247,458 Patented Apr. 19,1966 3,247.458 MONOSTABLE MULTIVIBRATOR EMPLOYING CAPACITIVE FEEDBACKCIRCUIT MAIN- TAINING VOLTAGE ACROSS INPUT CIR- CUIT SUBSTANTIALLYCONSTANT Atherton Noyes, .lr., Mountain Lakes, N.J., assignor toAircraft Radio Corporation, Boonton, N.J., a corporation of New JerseyOriginal application July 1, 1948, Ser. No. 36,494. pow Patent No.3,087,152, dated Apr. 23, 1963. Divided and this application Sept. 18,1962, Ser- No. 229,515 6 Claims. (Cl. 328-207) This invention relates toa multivibrator or shaper circuit and more particularly a one-shotmultivibrator circuit. This application is a division of my applicationSerial No. 36,494, filed July 1, 1948, now Patent 3,087,- 152, issuedApril 23, 1963.

An object of thisinvention is to provide an improved one-shotmultivibrator circuit.

Another object of this invention is to provide a circuit for developingsubstantially uniform magnitude duration signal pulses from input pulsesof varying intensity or amplitude.

The invention features a pair of amplifying elements with circuitelements connecting said amplifying elements for one-shot multivibratoroperation and in which the means for determining the duration of thestroke of the multivibrator is not directly in the cross-connectioncircuitry between the control and output electrodes of the respectiveamplifying elements. The invention also features a feedback connectionfor obviating the effect of varying signal input strength on the strokeof the multivibrator.

Other objects, advantages and features of the invent-ion will. becomeapparent from the following specification taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is an illustrative embodiment of a multivibrator circuitincorporating the invention; and V FIGS. 2A through 2D, inclusive, aretypical curves showing wave forms developed by weak and strong signalpulses, respectively, in the operationof the circuit.

With reference to FIG. 1, the input circuit com-prises a triode 30having a grounded cathode and a grid upon which a wave form comprisingnoise and signal peaks 1,

2 and 3 is impressed from the amplifier 12 through a coupling condenser31.

30 suppresses noise and clutter components, and the tube w 30 outputconsists of three negative voltage peaks 1, '2 and 3 which are developedacross output resistor 35.

The shaper is a novel one-shot mu-ltivibrator comprising triodes 36A,36B which may be, as shown schematically,

enclosed in a single envelope, and which has approximately a 2microsecond duration and with constants such that it is able to repeatrapidly. The input to triode 36A is through a condenser 37 connectedbetween its grid and the plate of tube 30, and triode 36A is normallyconductive in view of a positive voltage imposed on the .grid throughresistor 38, of the order of 400,000 ohms, which is connected to a pointof high positive voltage, for example to the +340 volt source. The plateof triode 36A is connected to the grid of triode 368 through a resistor40 and relatively large isolating condenser 41, and the grid of triode36B is normally biased negatively to cut-off by resistor 42 whichconnects the grid to the junction of voltage-divider resistors 43, 44 inseries between the negative potential bus 34 and ground.

As distinguished from conventional multivibrators, the

circuit elements which deter-mine the duration of the stroke are notdirectly in the feedback paths from plates to opposite grids. In thiscircuit the duration-determining constants are the input couplingcondenser 37 and the resistor 38; in conventional multivibrators of thistype, the duration-determining constants would be connected directlyfrom one plate to the opposite grid, and the input trigger signalconnected to grid, or plate, in parallel thereto.

Whereas, in conventional multiv-ibrators, the feedback arising from theoperation of the circuit is applied directly to a grid, which, in turnmay in some instances be also the grid to which the input trigger isapplied by way of some coupling impedance, in this circuit the feedbackis applied directly to the trigger source itself :and is utilized tomaintain the voltage output of the trigger source at a suitable value,regardless of what value it would have had in the absence of thisfeedback.

In this way the elements which determine the duration of themultivibrator stroke (namely the condenser 37 and the resistor 38) areenabled to operate under constant conditions, and thereby to maintainthe multivibrator stroke itself constant.

It is apparent from the above, of course, that the multivibrator must bea stiff source for feedback relative to the plate circuit of tube 30 asa load; this condition is readily achieved, for the impedance of theplate circuit of tube 36B during the stroke is very low due to thepositive voltage on its grid during this period.

The sequence of operations in the circuit is as follows: A negativevoltage pulse imposed on the grid of tube 36A by way of condenser 37from tube 30 causes a decrease in the plate current of tube 36A. Theresulting rise in plate voltage is carried, by Way of resistor 40 andlarge condenser 41, to the grid of tube 363 where it causes platecurrent to flow. The resulting drop in plate voltage of tube 36B is fedback to the plate circuit of tube 30 by way of large condenser 45, andthereby amplifies the original trigger by regeneration at this point.Tube 36A is thus carried far below cut-off, and tube 36B is caused toconduct very heavily. At this instant, and for the duration of thecycle, the high side of the input circuit, i.e. the .plate of tube 30,is depressed in voltage by the same amount as the fall in voltage of theplate of tube 36B, for condenser 45 is so large (for instance, 1000micromicro- 20 micromicrofarads) and therefore charges rapidly towardthe supply voltage (340 v0lts,for instance, as shown on FIG. 1). As soonas'the voltage onthe grid side of condenser 37 rises about the cut-offvoltage of tube 36A this tube begins to draw current again, feedbacks ofthe opposite sense arise, as is well known in the art, and the cycleends abruptly. The feedback through the large condenser 45 thus acts tostabilize the length of the cycle,

independent of the wave shape, amplitude and duration of the triggeringpulses at the tube 30, by maintaining the voltage on the left end sideof condenser 37 at a constant value. The input circuit condenser 37 isthereby permitted to charge under uniform conditions, regardless of theform of the trigger signal, until triode 36A is once more conductive andends the cycle.

The typical qualitative voltage curves of Figs. 2A to 2D illustrate themanner in which the described shaper circuit operates to develop outputpulses which are essentially independent of the amplitude and durationof the incoming triggers. In Fig. 2A, the curves 30g and 30'g show theform of positive voltage pulses imposed upon the grid of tube 30 bydelatively weak and relatively strong signals, respectively; except forthe feedback path above noted the plate voltage waves would differ fromeach other in similar fashion. However, in the actual circuit theresultant voltage waves 30p and 30p at the plate of tube 30 are as shownin Fig. 2B. It is to be noted that the drop in plate voltage for anarriving weak signal includes an initial section, indicated by thelegend Input Signal, and a supplemental and further voltage drop due tofeed back through condenser 45 and identified by legend Feedback. Theresultant variations in grid potential at tube 36A are shown in Fig. 2Cby curves 36g and 36 g for weak and strong signals, respectively. Theinputs to the grid of shaper tube 36A being thus substantially constantfor both weak and strong signals, the significant output voltage pulsesfrom the shaper are thus of a shape and timing substantially independentof the amplitude and duration of the arriving signal pulses.

It is to be noted that a very strong input trigger which has beenlengthened by overloads to a duration longer than the desired shaperstroke (right hand side of Fig. 2A) produces a corresponding lengtheneddisturbance in the plate of tube 30 (right hand side of Fig. 2B) butthat no corresponding lengthening is caused in the shaper grid circuit(right hand side of Fig. 2C) or in the shaper plate-voltage waves, 36por 36'p (Fig. 2D) which have the same duration as this grid wave.Conversely, a weak and short input trigger which, in the absence offeedback, would tend with its trailing edge to give a positive impulseto the grid of tube 36A (as indicated by dotted line in Fig. 2B) andthus end the stroke prematurely, is prevented from so doing by feedbackapplied in accordance with this invention, (see the left hand side ofFig. 2B).

Positive square wave output pulses may be taken off of the plate of tube36A, while negative square pulses corresponding to the incoming triggersmay be taken off the plate of tube 36B as shown in the drawings.

It is to be understood that the disclosed circuit is but oneillustrative embodiment of a multivibrator or shaper circuitincorporating the invention and is, therefore, not limited to thecircuit and circuit elements herein shown and described since variouschanges may be made without departing from the scope of theinvention asset forth in the following claims.

I claim:

1. A shaper circuit for developing square wave output signals ofsubstantially constant amplitude and duration from triggering signals ofwidely varying amplitude and duration, said shaper circuit including apair of tubes each having a control grid and plate cooperating with acathode, means operatively connecting said pair of tubes in said shapercircuit, means connected to the grid of a first of said tubes normallybiasing said first of said tubes to render that tube normallyconductive, means connected to the grid of a second of said tubes;normally biasing the said second of said tubes to render that tubenormally non-conductive, means connecting the plate of the normallyconductive tube to the grid of the second tube to render the latterconductive upon non-conduction of said normally conductive tube, aninput circuit, a low im- (50 pedance capacitive feedback circuit fromthe plate of said second and normally non-conductive tube to said inputcircuit, input means for developing a trigger voltage across said inputcircuit, means comprising said feedback circuit for maintaining thevoltage across said input circuit substantially constant during the timesaid second tube is conductive, and timing means connecting the grid ofsaid first and normally conductive tube to said input circuit and to asource of positive voltage for establishing the duration of saidconducting period of said second tube.

2. A shaper circuit as recited in claim 1, wherein said input meansincludes means responsive to arriving trigger signals of varyingamplitude and duration to develop across said input circuit a triggervoltage component of similarly varying amplitude and duration.

3. A shaper circuit as recited in claim 1, wherein said timing meanscomprises a coupling condenser of low capacity connected between thegrid of the normally conductive tube and said input circuit, and saidcapactive feedback circuit is connected from the plate of the normallynonconductive tube to the input side of said coupling condenser.

4. A shaper circuit as recited in claim 3, wherein said timing meansincludes a resistor of high value connected between the grid of saidnormally conductive tube and the source of positive voltage.

5. In a monostable multivibrator circuit having input and outputterminals, said multivibrator having an unstable phase of operationinitiated by a trigger signal applied to said input terminal,

a timing circuit including a series connected capacitance and resistanceand a high potential source connected to said resistance, 7

means connecting the common point of said resistance and capacitance tosaid input terminal,

a low impedance capacitance feedback circuit connected between saidoutput terminal and the opposite side of said capacitance from the sideconnected to said resistance for maintining the potential at saidopposite side substantially constant during the unstable phase ofoperation of said multivibrator,

and means connected to said multivibrator circuit for applying triggerpulses to said multivibrator circuit through said capacitance toinitiate said unstable phase of operation.

6. In a monostable multivibrator circuit,

a timing circuit for said multivibrator including a capacitor,

means connecting said capacitor between said multivibrator circuit and asource of trigger pulses so that said capacitor passes trigger pulses tosaid multivibrator circuit,

and a capacitive feedback circuit connected between an output terminalof said multivibra'tor and the side of said capacitor connected'to saidsource of trigger pulses for maintaining the potential at said sideconnected to said source of trigger pulses substantially constant duringthe unstable state of said multivibrator. 1 1

References Cited by the Examiner UNITED STATES PATENTS 2,688,079 8/1954Wachtell 328-207 ARTHUR (jAUSS, Primary Examiner.

1. A SHAPER CIRCUIT FOR DEVELOPING SQUARE WAVE OUTPUT SIGNALS OFSUBSTANTIALLY CONSTANT AMPLITUDE AND DURATION FROM TRIGGERING SIGNALS OFWIDELY VARYING AMPLITUDE AND DURATION, SAID SHAPER CIRCUIT INCLUDING APAIR OF TUBES EACH HAVING A CONTROL GRID AND PLATE COOPERATING WITH ACATHODE, MEANS OPERATIVELY CONNECTING SAID PAIR OF TUBES IN SAID SHAPERCIRCUIT, MEANS CONNECTED TO THE GRID OF A FIRST OF SAID TUBES NORMALLYBIASING SAID FIRST OF SAID TUBES TO RENDER THAT TUBE NORMALLYCONDUCTIVE, MEANS CONNECTED TO THE GRID OF A SECOND OF SAID TUBES;NORMALLY BIASING THE SAID SECOND OF SAID TUBES TO RENDER THAT TUBENORMALLY NON-CONDUCTIVE, MEANS CONNECTING THE PLATE OF THE NORMALLYCONDUCTIVE TUBE TO THE GRID OF THE SECOND TUBE TO RENDER THE LATTERCONDUCTIVE UPON NON-CONDUCTION OF SAID NORMALLY CONDUCTIVE TUBE, ANINPUT CIRCUIT, A LOW IMPEDANCE CAPACITIVE FEEDBACK CIRCUIT FROM THEPLATE OF SAID SECOND AND NORMALLY NON-CONDUCTIVE TUBE TO SAID INPUTCIRCUIT, INPUT MEANS FOR DEVELOPING A TRIGGER VOLTAGE ACROSS SAID INPUTCIRCUIT, MEANS COMPRISING SAID FEEDBACK CIRCUIT FOR MAINTAINING THEVOLTAGE ACROSS SAID INPUT CIRCUIT SUBSTANTIALLY CONSTANT DURING THE TIMESAID SECOND TUBE IS CONDUCTIVE, AND TIMING MEANS CONNECTING THE GRID OFSAID FIRST AND NORMALLY CONDUCTIVE TUBE TO SAID INPUT CIRCUIT AND TO ASOURCE OF POSITIVE VOLTAGE FOR ESTABLISHING THE DURATION OF SAIDCONDUCTING PERIOD OF SAID SECOND TUBE.